Plastics are used as product materials in all fields of daily life, and the amount of plastics used is increasing each year. Accompanying this increase, the amount of discarded plastics is also extremely large, thus resulting in the treatment of plastics becoming a significant social issue.
At present, the majority of plastic products are simply disposed of by being incinerated or buried following completion of their use. However, when waste plastic having high heat of combustion in terms of calories is disposed of by incinerating in an ordinary refuse incinerator, abnormal combustion occurs resulting in the problem of damage to the incinerator furnace. In addition, not only does this manner of disposal result in wasted resources, but it also causes environmental problems in terms of environmental contamination and discharge of carbon dioxide gas. Thus, it is extremely important to recycle waste plastics from the viewpoint of the formation of a recycling society as well.
Methods used to recycle waste plastics include material recycling, in which waste plastics are reused as is, chemical recycling, in which waste plastics are chemically degraded followed by recovery of monomers and other useful chemical raw materials, and thermal recycling, in which thermal energy is recovered from waste plastics. Among these, since material recycling is accompanied by heat treatment of the waste plastics, the heat treatment has a considerable effect on both the chemical properties and physical properties of the waste plastics, and frequently results in problems such as deterioration of impact resistance, deformation under a load or high temperatures, tensile strength, bending strength, fluidity and other properties. In addition, although thermal recycling offers the advantage of being able to inhibit the amount of fossil fuels used as a result of effectively utilizing thermal energy, there are also numerous problems such as damage to the incinerator furnace, discharge of carbon dioxide gas and the need to implement measures against dioxins as described above.
Aromatic polycarbonate resins constitute a typical engineering plastic having superior transparency, optical properties and mechanical properties, and are extremely high added value materials used in a wide range of applications such as CDs, DVDs and other optical fields, various home appliances, cameras, cell phones, OA equipment, medical equipment, automobiles and other industrial fields, sports and other recreational fields, and roofing materials, alternative glass materials and other construction fields.
Various methods have been proposed thus far for chemically recycling aromatic polycarbonates.
According to Non-patent document 1, although a process is described for obtaining bisphenol A by chemically decomposing polycarbonate resin with ammonia water, decomposition of the polycarbonate resin requires a long period of time, thereby resulting in the problem of being unsuitable for large-volume processing of waste plastics.
In addition, Patent document 1 discloses a process for recovering bisphenol A by decomposing polycarbonate resin by adding ammonia water and an organic solvent in the form of aluminum chloride to a polycarbonate resin. However, there are many cases in which chemical decomposition of the polycarbonate requires a long period of time with this process as well.
Examples of processes for shortening the time required to decompose polycarbonate resins in this manner may include a process for recovering useful materials from waste plastics having polycarbonate resin for the main component thereof disclosed in Patent document 2 which comprises a step of chemically decomposing a polycarbonate resin in a solution containing waste plastic and a decomposition agent in the form of a primary amine, and a step of recovering the decomposition product in the form of a useful material. In this process, the polycarbonate resin is reacted with an excess of primary amine equivalent to six or more times the number of moles of carbonic acid ester groups as calculated from the molecular weight of the repetitive units of the polycarbonate resin, followed by recovery of useful materials such as the degradation product in the form of bisphenol A and urea derivatives. In addition, according to Non-patent document 2, it is described that bisphenol A and 1,3-dimethyl-2-imidazolidinone (DMI) are obtained by decomposing polycarbonate with N,N′-dimethyl-1,2-diaminoethane. Among these recovery products, although bisphenol A can be easily imagined to be used as a raw material for the production of polycarbonate resin, there are no descriptions regarding the use of urea derivatives or DMI, and the usefulness thereof is unclear.
In addition, according to Patent document 3, for example, a process is disclosed for obtaining bisphenols and diaryl carbonate by cleaving polycarbonate resin by carrying out a transesterification reaction between polycarbonate resin and phenol in the presence of a catalyst. It is described to the effect that monomers obtained by this process can be recondensed to produce polymer plastics. In addition, in Patent document 4, for example, a process for recovering useful materials from waste plastics mainly composed of polycarbonate is disclosed whereby decomposition products are recovered in the form of useful materials by chemically decomposing polycarbonate resin in a solution containing an organic solvent that causes polycarbonate resin to dissolve or swell, a tertiary amine and a lower alcohol. In this process, examples of recovered useful materials are listed as being bisphenol A and carbonic acid ester. Since each of these processes requires an alkaline catalyst to decompose the polycarbonate by a transesterification reaction, there are many cases in which the procedure becomes complex, such as requiring deactivation of the alkaline catalyst during separation and purification of the decomposition products.
As an example of a process not requiring a catalyst, Non-patent document 3 discloses a process for producing bisphenol A by hydrolyzing polycarbonate under supercritical conditions (supercritical aqueous or subcritical aqueous conditions). Although there is no description regarding yield and the reaction efficiency is not clearly stated in this document, since the reaction is carried out under high temperature and high pressure conditions, not only is there the possibility of the concurrent occurrence of thermal decomposition of the bisphenol A under such conditions, but also due to the extremely strong acidity of the water itself under supercritical aqueous conditions along with the high temperature in excess of 300° C. and high pressure in excess of 200 atm, the apparatus and equipment become excessively complex, thereby making it difficult to carry out the process economically.
Patent document 5 discloses a process for recovering aromatic bisphenol and carbonic acid ester formed by reacting polycarbonate obtained by melting and filtration from disk-shaped optical recording media with an aliphatic alcohol having 1 to 6 carbon atoms in a subcritical or supercritical state. In this process, in addition to the reaction vessel being large since an excess of aliphatic alcohol is used based on the polycarbonate, similar to the case of the process described in Non-patent document 3, since the reaction vessel is required to be of a design capable of withstanding a high temperature and high pressure state, the large reactors used in typical commercial plants encounter difficulties both in terms of design and economy.
Although polycarbonates have a typical structure in which, for example, a bisphenol A unit and a carbonyl unit are alternately arranged in a polymer chain, the chemical recycling processes disclosed thus far disclose technologies that only attempt to effectively recycle one of these units or technologies that only attempt to recover the bisphenol A. However, there have been no successful examples of chemically recycling both units in the form of effective compounds at a high recovery yield.
Thus, although there has been a strong desire for the development of a process for chemically recycling waste aromatic polycarbonate resins, an effective process has yet to be found.
As previously described, polycarbonate resins are formed from, for example, bisphenol A and carbonyl units. The recovery of this carbonyl unit in the form of an industrially effective compound is an important issue for chemical recycling of polycarbonate resins. Examples of industrially effective compounds having a carbonyl group may include carbonic acid esters and isocyanates. Isocyanates are widely used as production raw materials of polyurethane foam, paints and adhesives. The most commonly used process for industrial production of isocyanates consists of reacting an amine compound with phosgene (phosgene method), and nearly the entire amount of isocyanates produced throughout the world are produced according to the phosgene method. However, the phosgene method has numerous problems.
Firstly, this method requires the use of a large amount of phosgene as raw material. Phosgene is extremely toxic and requires special handling precautions to prevent exposure of handlers thereof, and also requires special apparatuses to detoxify waste.
Secondly, since highly corrosive hydrogen chloride is produced in large amounts as a by-product of the phosgene method, in addition to requiring a process for detoxifying the hydrogen chloride, in many cases hydrolytic chlorine is contained in the isocyanates produced, which may have a detrimental effect on the weather resistance and heat resistance of polyurethane products in the case of using isocyanates produced using the phosgene method.
On the basis of this background, a process for producing isocyanates has been sought that does not use phosgene. One example of a method for producing isocyanate compounds without using phosgene that has been proposed involves thermal decomposition of carbamic acid esters. Isocyanates and hydroxy compounds have long been known to be obtained by thermal decomposition of carbamic acid esters (see, for example, Non-patent document 4). The basic reaction is illustrated by the following formula:R(NHCOOR′)a→R(NCO)a+aR′OH  (1)(wherein R represents an organic residue having a valence of a, R′ represents a monovalent organic residue, and a represents an integer of 1 or more).
Among carbamic acid esters, aryl carbamates, in which the ester group is an aromatic group, offer the advantage of allowing the setting of a lower temperature for the thermal decomposition reaction as compared with alkyl carbamates in which the ester group is an alkyl group (see, for example, Patent document 6).
Various processes have been disclosed thus far as processes for producing aryl carbamates. Patent document 7 describes the obtaining of a corresponding alkyl aryl monocarbamate at a yield of 90 to 95% by reacting an alkyl monoamine and a diaryl carbonate in the presence of a solvent such as benzene, dioxane or carbon tetrachloride. In addition, Patent document 8 proposes a process for continuously producing methyl carbamic acid phenyl ester from methylamine and diphenyl carbonate.
However, each of these processes produces alkyl aryl carbamate using a lower alkyl monoamine for the amine, and do not constitute a process for producing an alkyl aryl polycarbamate. In the case of producing a corresponding alkyl polycarbamic acid aryl ester from an alkyl polyamine such as alkyl diamine or alkyl triamine, there are difficult problems that are completely different from those in the case of using an alkyl monoamine. This is because, although only urea compounds are produced as by-products due to side reactions represented by formula (3) and/or formula (4) in addition to the reaction represented by formula (2) in the case of an alkyl monoamine, in the case of an alkyl polyamine such as alkyl diamine or alkyl triamine, an extremely large number of types of urea compounds are produced as by-products, such as the compounds represented by formula (5) and/or formula (6) and/or formula (7).
(wherein R′ represents a monovalent alkyl group or aromatic group, Ar represents a monovalent aromatic group, and p, q and r respectively represent an integer of 1 or more.)
Namely, there are the problems of these various urea compound side reactions causing a decrease in the yield of the target compound in the form of the alkyl aryl polycarbamate, as well as the extreme difficulty in separating and purifying the target product from a mixture of these urea compounds and polyurea compounds.
Although Patent document 9 describes a process for synthesizing an aromatic urethane by reacting an aromatic amine and a diaryl carbonate in the presence of a Lewis acid catalyst at a temperature of 140 to 230° C., in this process as well, the use of a Lewis acid results in the problem of corrosion of the apparatus as well as difficulty in separating and recovering the product.
Patent document 10 discloses a process for producing alkyl aryl polycarbamate comprising the use of 1 to 3 equivalents of diaryl carbonate based on 1 equivalent of amino group of alkyl polyamine, using an aromatic hydroxy compound for the reaction solvent, and carrying out the reaction in the state of a substantially homogeneous solution when producing alkyl polycarbamic acid aryl ester by reacting alkyl polyamine and diaryl carbonate. According to this patent document, alkyl polycarbamic acid aryl ester is obtained with high selectivity and at a high yield of normally 96% or more and 98% or more in preferable embodiments thereof. However, since the formation of urea compounds has been confirmed, although in very small amounts, this process is unable to completely avoid the formation of urea compounds.
On the other hand, thermal decomposition of carbamic acid esters is susceptible to the simultaneous occurrence of various irreversible side reactions such as thermal denaturation reactions undesirable for carbamic acid esters or condensation of isocyanates formed by the thermal decomposition. Examples of these side reactions may include a reaction in which urea bonds are formed as represented by the following formula (8), a reaction in which carbodiimides are formed as represented by the following formula (9), and a reaction in which isocyanurates are formed as represented by the following formula (10) (see, Non-patent document 4 and Non-patent document 5).
(wherein R and R′ independently represent monovalent alkyl groups or aromatic groups.)
In addition to these side reactions leading to a decrease in yield and selectivity of the target isocyanate, in the production of polyisocyanates in particular, these reactions may make long-term operation difficult as a result of, for example, causing the precipitation of polymeric solids that clog the reaction vessel.
Various processes have been proposed thus far for producing isocyanates using a carbamic acid ester for the raw material.
According to Patent document 11, an aromatic diisocyanate and/or polyisocyanate is produced by going through the following two steps. More specifically, in the first step, an aromatic primary amine and/or an aromatic primary polyamine is reacted with an O-alkyl carbamate in the presence or absence of a catalyst and in the presence or absence of urea and alcohol to form an aryl diurethane and/or aryl polyurethane followed by removal of the ammonia formed as necessary. In the second step, an aromatic isocyanate and/or aromatic polyisocyanate are obtained by thermal decomposition of the aryl diurethane and/or aryl polyurethane.
There are several known methods for forming a corresponding isocyanate and alcohol by thermal decomposition of a (cyclic) aliphatic, and particularly an aromatic, monourethane and diurethane, including methods carried out in the gaseous phase at a high temperature, and methods carried out in a liquid phase under comparatively low temperature conditions. In these methods, however, the reaction mixture gives rise to the side reactions described above, thereby causing, for example, the formation of sediment, polymeric substances and obstructions in the reaction vessel and recovery apparatus, or the formation of substances that adhere to the sidewalls of the reaction vessel, thereby resulting in poor economic efficiency in the case of producing isocyanates over a long period of time.
Thus, the use of a chemical method, such as the use of a special catalyst (see, for example, Patent document 12 and Patent document 13) or a catalyst combined with an inert solvent (see, for example, Patent document 14), has been disclosed to improve the yield in the thermal decomposition of urethane.
More specifically, Patent document 15 describes a process for producing hexamethylene diisocyanate consisting of thermal decomposition of hexamethylene diethyl urethane in the presence of dibenzyl toluene used as a catalyst and in the presence of a catalyst mixture composed of methyl toluene sulfonate and diphenyl tin dichloride. However, since there are no detailed descriptions of production or isolation of the starting components or purification and arbitrary recovery of the solvent and catalyst mixture, the economic efficiency of this process could not be evaluated.
According to the method described in Patent document 16, urethane can be easily broken down into isocyanate and alcohol in a carbon-containing fluidized bed without using a catalyst. In addition, according to the description of Patent document 17, hexamethylene dialkyl urethane can be decomposed in the gaseous phase at a temperature in excess of 300° C. in the presence or absence of a gas permeable packaging material made of, for example, carbon, copper, brass, steel, zinc, aluminum, titanium, chromium, cobalt or quartz to form hexamethylene diisocyanate. According to the description of Patent document 16, this method is carried out in the presence of a hydrogen halide and/or hydrogen halide donor. However, this method is unable to achieve a yield of hexamethylene diisocyanate of 90% or more. This is because the decomposition products are partially recombined resulting in the formation of urethane bonds. Thus, the hexamethylene diisocyanate is required to be additionally purified by distillation, which frequently increases yield loss.
Moreover, according to the description of Patent document 18, a monocarbamate is disclosed to be able to be decomposed with favorable yield without using a solvent and in the presence or absence of a catalyst and/or stabilizer at a comparatively low temperature and advantageously under a reduced pressure. The decomposition products (monoisocyanate and alcohol) are removed from a boiling reaction mixture by distillation and separately captured by separative condensation. A method for removing by-products formed during thermal decomposition consisting of partially removing the reaction mixture outside the system is described in a generic form. Thus, although by-products can be removed from the bottom of the reaction vessel, problems remain with respect to the case of adherence to the sidewalls of the reaction vessel as previously described, and problems with respect to long-term operation remain unsolved. In addition, there is no description regarding industrial utilization of the removed reaction mixture (containing large amounts of useful components).
According to the description of Patent document 19, thermal decomposition of an aliphatic, alicyclic or aromatic polycarbamate is carried out in the presence of an inert solvent at 150° C. to 350° C. and 0.001 to 20 bar, and in the presence or absence of a catalyst and an assistant in the form of hydrogen chloride, organic acid chloride, alkylation agent or organic tin chloride. By-products formed, can be continuously removed from the reaction vessel together with the reaction solution, for example, and a corresponding amount of fresh solvent or recovered solvent is added simultaneously. A shortcoming of this method is, for example, a reduction in the space-time yield of polyisocyanate as a result of using a refluxing solvent, while also requiring considerable energy, including that for recovery of the solvent. Moreover, the assistants used are volatile under the reaction conditions, resulting in the potential for contamination of the decomposition products. In addition, the amount of residue is large based on the formed polyisocyanate, thus leaving room for doubt regarding economic efficiency and the reliability of industrial methods.
According to the description of Patent document 20, a method is described for continuous thermal decomposition of a carbamate such as the alicyclic diurethane, 5-(ethoxycarbonylamino)-1-(ethoxycarbonylaminomethyl)-1,3,3-trimethylcyclohexane, supplied along the inner surface of a tubular reactor in a liquid form in the presence of a high boiling point solvent. This method has the shortcomings of low yield during production of (cyclic) aliphatic diisocyanates and low selectivity. In addition, there is no description of a continuous method accompanying recovery of recombined or partially decomposed carbamates, while there is also no mention made of post-treatment of solvent containing the by-products and catalyst.    Patent document 1: Japanese Patent Application Laid-open No. H6-25086    Patent document 2: Japanese Patent Application Laid-open No. 2003-231774    Patent document 3: Japanese Patent Application Laid-open No. H6-56985    Patent document 4: Japanese Patent Application Laid-open No. 2002-212335    Patent document 5: Japanese Patent Application Laid-open No. 2004-339147    Patent document 6: U.S. Pat. No. 3,992,430    Patent document 7: Japanese Patent Application Laid-open No. S52-71443    Patent document 8: Japanese Patent Application Laid-open No. S61-183257    Patent document 9: Japanese Patent Application Laid-open No. 2004-262834    Patent document 10: Japanese Patent Application Laid-open No. H1-230550    Patent document 11: U.S. Pat. No. 4,290,970    Patent document 12: U.S. Pat. No. 2,692,275    Patent document 13: U.S. Pat. No. 3,734,941    Patent document 14: U.S. Pat. No. 4,081,472    Patent document 15: U.S. Pat. No. 4,388,426    Patent document 16: U.S. Pat. No. 4,482,499    Patent document 17: U.S. Pat. No. 4,613,466    Patent document 18: U.S. Pat. No. 4,386,033    Patent document 19: U.S. Pat. No. 4,388,246    Patent document 20: U.S. Pat. No. 4,692,550    Non-patent document 1: Polymer Chemistry, Vol. 20, No. 214, 1963    Non-patent document 2: the Collection of Preliminary Manuscripts of the Study Group of the Research Association for Feedstock Recycling of Plastics, Vol. 3, pp. 31-32, 2001    Non-patent document 3: Polymer Preprints, Japan, Vol. 54, No. 1, 2005    Non-patent document 4: Berchte der Deutechen Chemischen Gesellschaft, Vol. 3, p. 653, 1870    Non-patent document 5: Journal of American Chemical Society, Vol. 81, p. 2138, 1959